Recently, with advancements in magnetic recording, a higher image quality and a higher tone quality have been demanded. For meeting this demand, proposals have been pursued including fining the particle size of a ferromagnetic particle for magnetic recording media and increasing the recording density of the magnetic recording media. Also, with the increase in the demand for and use of magnetic recording media, it nonetheless has been required to produce the magnetic recording media at an even more competitive cost.
As one technique, the employment of a double layer (i.e., a multi-layer) type magnetic recording medium is known. The double layer is advantageous for increasing high-density recording while balancing other properties since suitable ferromagnetic particles can be used for imparting high image quality characteristics to the upper layer and high tone quality characteristics to the lower layer. Also, the employment of the double layer type magnetic recording medium has another salutary feature that the magnetic recording medium can be produced at a low cost since proper materials can be used for each layer. At present, a method of improving the surface property of the surface of the magnetic layer, in general, is considered to be effective to increase the electromagnetic properties.
On the other hand, the electromagnetic properties of a magnetic recording medium are the properties which shall be evaluated on the assumption that the magnetic recording medium has a good running performance above a prescribed level. Hence, in a magnetic recording medium, for ensuring the good running performance, it is necessary that the friction coefficient of the magnetic layer surface and the back layer surface is relatively small.
Accordingly, at present, a magnetic recording medium is required to have all of (1) excellent electromagnetic properties, (2) an excellent running durability, and (3) excellent productivity.
For attaining the foregoing requirement (1), it is necessary to smoothen the surface of the magnetic layer as smooth as possible. For this purpose, it is required that the surface of the base (support) at the side of coating the magnetic layer is very smooth. In particular, in the case of the magnetic layer of the double layer structure, the surface property of the 1st magnetic layer (hereinafter, referred to as a lower layer) determines the surface property of the whole recording tape. Hence, the surface property of the base support, which gives a large influence on the surface property of the lower layer is likewise important and is required to be smooth.
Then, for improving the running durability, which is the 2nd requirement mentioned above, it is necessary to lower the friction coefficient of the surface of the magnetic layer and the surface of the backing layer. Usually, the running property of a magnetic recording tape is determined by the sliding property between the surface of the magnetic layer of the magnetic recording tape and a head, cylinders, etc., and the sliding property between the back surface of the magnetic recording tape and poles, guides, etc., in a magnetic tape deck. If the friction coefficient is high, there occur problems of stick slip, sticking, and so forth. Thus, for lowering the friction coefficient of the surface of the magnetic layer, it is known to add a lubricant into or on the magnetic layer or topically applying a lubricant onto the surface of the magnetic layer.
Also, as to the back surface, if the back surface of the support, as is, of a magnetic recording tape is simply slided with a guide and a pole, a good friction coefficient is not obtained. Thus, it is known to increase the unevenness of the surface of the back surface and to form a back coat layer thereon. However, if the unevenness of the surface of the back layer is too large, the roughness is sometimes transferred onto the surface of the magnetic layer when the magnetic recording tape is wound to deteriorate the electromagnetic properties of the magnetic recording tape. Accordingly, it is necessary that the surface of the back layer does not have a surface roughness of an extent of being transferred onto the surface of the magnetic layer, but instead has a proper surface roughness capable of ensuring the good running property.
Furthermore, as to the excellent productivity which is the 3rd above mentioned requirement, it is required to obtain a good running durability by not only employing simply a double layer for the magnetic layer, but, also, without forming a back layer.
Previously, JP-A-63-175222 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a base for a magnetic recording medium, which can prevent the occurrence of the deformation of projections of the base, keep the friction coefficient of the base at a constant, and does not deteriorate the electromagnetic properties of the magnetic recording medium by using two kinds of spherical silicas as the fillers for the base and making the state of the projections of the fillers denser than a certain extent.
However, the surface of the foregoing base is not sufficiently smooth since the base contains two kinds of fillers, each having a different size. Hence, when a magnetic layer of a double layer structure is formed on the base, the smoothness of the lower magnetic layer is insufficient and the unevenness of the lower layer is amplified to form a rough magnetic layer as the upper layer. Thus, the electromagnetic properties are not desirably improved at the surface side of the magnetic layer and also the large fillers exposed locally are scratched at the back surface side of the base, whereby the durability is liable to deteriorate.
Accordingly, the back surface of the foregoing base can not be used unless provided with a back coating layer.
Thus, for the purpose of providing a magnetic recording medium having a high productivity, wherein the base surface is smooth and the magnetic layer formed thereon has excellent electromagnetic properties, as well as the magnetic recording medium has a good running durability at the back surface without the need of a back coating layer, one proposal is a magnetic recording medium comprising a polyethylene terephthalate base having formed on the surface thereof a 1st magnetic layer and a 2nd magnetic layer, in this order, wherein the polyethylene terephthalate base contains inert inorganic particles having a particle size distribution of a mean particle size of from 0.5 to 0.7 .mu.m and a standard deviation of not larger than 0.1 .mu.m, the center line average roughness (Ra) of the surface of the polyethylene terephthalate base is not more than 16 nm, and the number of projections having a projection density (Ds) of at least 0.7 .mu.m is at least 6,000/mm.sup.2 (i.e., at least 60 per 0.1 mm.sup.2), as disclosed in JP-A-2-193313.
However, recently, the degree of a high image quality and a high tone quality required has been ever increased. With the increase demands, the improvement of the electromagnetic properties, in particular, fining of the particle size of a ferromagnetic particle and the increase of high-density recording have been considered. Further, it is required to concomitantly lower the occurrence of noise and increase C/N by smoothening the surface of the magnetic recording tape. However, for achieving such requirements, sufficiently satisfactory results are not assurable by the foregoing proposal.
If the surface of a magnetic recording medium becomes smooth, the friction coefficient is increased to reduce the running durability. On the other hand, if the friction coefficient of a magnetic recording medium is large, the magnetic layer and the back surface thereof are damaged by repeated running to form abrasions and scratched particles which attach to the magnetic layer to cause drop out. Also, if the electrostatically charging property of a magnetic recording tape is large, dust and scratched particles are liable to attach to the surface of the magnetic layer during running of the tape to cause drop out. For obtaining a stable running property and preventing the increase of drop out by repeated use of a magnetic recording tape, it is necessary that the friction coefficient of the magnetic recording layer and the back surface is low and further damage generated by sliding with the parts in a tape cassette and guide pins in a tape deck is prevented, and that the charging property of the magnetic recording tape is low.
For these requirements, it has hitherto been practiced to roughen the surface property of the back surface of a magnetic recording medium for purposes of lowering the friction coefficient of the back surface and to incorporate carbon black into the magnetic layer for purposes of lowering the charging property thereof.
However, when the back surface is roughened, the roughness on the back surface is transferred onto the surface of the magnetic layer in the case of winding the magnetic recording tape to deteriorate the surface property of the magnetic layer and to lower the electromagnetic properties of the magnetic recording tape. Also, since carbon black is a non-magnetic particle, if a large amount of carbon black is incorporated in the magnetic layer, the magnetic characteristics are lowered to reduce the electromagnetic properties. Accordingly, it is difficult to realize both the requirement for lowering the electromagnetic properties and the friction coefficient and for lowering the charging property.
As a means for realizing both the requirements, it is proposed to form a back coating layer containing carbon black, and so forth, on the surface of the back surface of the support. However, by the foregoing means, the effect of realizing both the above mentioned requirements is insufficient. In particular, there is a problem that such a magnetic recording tape is inferior in sliding property with respect to resin guide pins recently being used in tape cassettes.